This application aims to elucidate the molecular mechanism by which the two non-enveloped viruses polyomavirus (Py) and SV40 penetrate the endoplasmic reticulum (ER) membrane. To infect cells, Py and SV40 bind to glycolipid receptors called gangliosides on the host cell surface and are internalized. The viruses are then transported to the lumen of the ER where they co-opt host cell machineries to cross the ER membrane and reach the cytosol. From the cytosol, Py and SV40 are transported into the nucleus where transcription and replication of the viral DNA ensue, leading to lytic infection or cell transformation. How these viruses penetrate the ER membrane to reach the cytosol, a decisive infection event, remains mysterious and is a process we intend to clarify in this proposal.